The present invention relates to a honeycomb structural body used in, for example, a catalyst carrier utilizing a catalytic action, for use in an internal combustion engine, a boiler, a chemical reactor, a fuel cell reformer, etc., and a filter for capturing fine particles present in an exhaust gas; as well as to an assembly thereof. More particularly, the present invention relates to a honeycomb structural body which has excellent durability against breakage caused by thermal stress appearing therein during its use, as well as to an assembly thereof.
Honeycomb structural bodies are in use in, for example, a carrier for a catalyst having a catalytic action, for use in an internal combustion engine, a boiler, chemical reactor, a fuel cell reformer, etc., and a filter for capturing fine particles present in an exhaust gas, particularly fine particles emitted from a diesel engine.
In the honeycomb structural body used for such a purpose, the sharp temperature change of exhaust gas or local heating makes non-uniform the temperature distribution inside the honeycomb structural body and there have been problems such as crack generation in the honeycomb structural body and the like. When the honeycomb structural body is used particularly as a filter for capturing a particulate substance in an exhaust gas emitted from a diesel engine, it is necessary to burn the fine carbon particles deposited on the filter to remove the particles and regenerate the filter and, in that case, high temperatures are inevitably generated locally in the filter; as a result, this process tends to generate large thermal stress and cracks.
Hence, there have been proposed processes for producing a honeycomb structural body by bonding a plurality of individual honeycomb segments using an adhesive. For example, U.S. Pat. No. 4,335,783 discloses a process for producing a honeycomb structural body, which comprises bonding a large number of honeycomb parts using a discontinuous adhesive. JP-B-61-51240 proposes a heat shock-resistant rotary regenerating heat exchanging method which comprises forming, by extrusion, matrix segments of honeycomb structural body made of a ceramic material, firing them, making smooth, by processing, the outer peripheral portions of the fired segments, coating the to-be-bonded areas of the resulting segments with a ceramic adhesive having, when fired, substantially the same mineral composition as the matrix segments and showing a difference in thermal expansion coefficient, of 0.1% or less at 800° C., and firing the coated segments. SAE article 860008 of 1986 discloses a ceramic honeycomb structural body obtained by bonding cordierite honeycomb segments with a cordierite cement. JP-A-8-28246 discloses a ceramic honeycomb structural body obtained by bonding honeycomb ceramic members with an elastic sealant made of at least a three-dimensionally intertwined inorganic fiber, an inorganic binder, an organic binder and inorganic particles.
Meanwhile, the regulation for exhaust gas has become stricter and engines have come to have higher performance. As a result, in order to achieve an improvement in combustion conditions of an engine and an increase in purification ability of a catalyst, the temperature of exhaust gas has increased year by year. In this connection, a higher thermal shock resistance has become required for honeycomb carriers. Therefore, even with honeycomb structural bodies such as mentioned above, when a sharp temperature change of inflow gas takes place, and a local heat of reaction, a local heat of combustion, etc., become larger during use, a thermal stress applied thereto may not be sufficiently relaxed, cracks may appear therein and, in an extreme case, there may occur, for example, disintegration of the honeycomb structural body and breakage of the structural body into fine pieces caused by vibration.
In order to solve these problems, there is a method of allowing a honeycomb structural body to have a large heat capacity, thereby making small the temperature change, and reducing the reaction rate and the combustion rate, lowering the maximum temperature, resulting in a relaxing of the thermal stress acting on the honeycomb structural body. Such a method, however, has had drawbacks of reductions in the reaction rate, purification efficiency and regeneration efficiency of a honeycomb structural body. In JP-B-54-110189 is proposed a honeycomb structural body in which the wall thickness of the honeycomb carrier is made smaller regularly in the cross-section toward the sectional center; and in JP-A-54-150406 and JP-A-55-147154 is proposed a honeycomb structural body in which the wall thickness of outer cells is made larger than the wall thickness of inner cells. These honeycomb structural bodies have large resistance to external mechanical stress; however, since the wall thickness of the inner cells is small, a large thermal stress is generated during use of the honeycomb structural body and its durability is not sufficient.
The present invention has been made in view of such past situations and aims at providing a honeycomb structural body which is low in reductions in the conversion rate, purification efficiency, regeneration efficiency, etc., during use and superior in durability against breakage caused by thermal stress.